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Research Papers: Elastohydrodynamic Lubrication

# Friction Reduction in Lubricated Rough Contacts: Numerical and Experimental Studies

[+] Author and Article Information
Zhiqiang Liu

Ford Motor Company,
Dearborn, MI 48121
e-mail: zhiqiangliu@yahoo.com

Ford Motor Company,
Dearborn, MI 48121

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received July 7, 2015; final manuscript received September 21, 2015; published online November 4, 2015. Assoc. Editor: Mircea Teodorescu.

J. Tribol 138(2), 021506 (Nov 04, 2015) (12 pages) Paper No: TRIB-15-1246; doi: 10.1115/1.4031752 History: Received July 07, 2015; Revised September 21, 2015

## Abstract

Combining the contact model of elastic-layered solid with the concept of asperity contact in elastohydrodynamic lubrication (EHL), a mixed-lubrication model is presented to predict friction coefficient over rough surfaces with/without an elastic-layered medium under entire lubrication regimes. Solution of contact problems for elastic-layered solids is presented based upon the classical model of Greenwood and Williamson (GW) in conjunction with Chen and Engel's analysis. The effects of the Young's modulus ratio of the layer to substrate and the thickness of the layer on the elastic real area of contact and contact load for a fixed dimensionless separation are studied using the proposed method, which is used for the asperities having contact with an elastic coating. Coefficient of friction with elastic-layered solids in boundary lubrication is calculated in terms of Rabinowicz's findings and elastic-layered solutions of Gupta and Walowit. The effect of rough contacts with an elastic layer on friction coefficient in lubrication regimes has been analyzed. Variations in plasticity index $ψ$ significantly affect friction coefficients in boundary and mixed lubrications. For a large value of $ψ$, the degree of plastic contact exhibits a stronger dependence of the mean separation or film thickness than the roughness, and for a small value of $ψ$, the opposite result is true. The effect of governing parameters, such as inlet oil viscosity at ambient pressure, pressure–viscosity coefficient, combined surface roughness, and $El/E2$ on friction coefficient, has been investigated. Simulations are shown to be in good agreement with the experimental friction data.

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## Figures

Fig. 1

Normalized pressure distribution based on GW, KE, and JG models

Fig. 2

(a) The geometry of contacting rough surfaces and (b) contact of a spherically capped summit and a plane

Fig. 3

Contact with an elastic layer

Fig. 4

Pressure–viscosity coefficient

Fig. 5

Elastic real area of contact versus coating thickness for contact with an elastic layer in a range of the ratio El/E2

Fig. 6

Elastic contact load versus coating thickness for contact with an elastic layer in a range of the ratio El/E2

Fig. 7

Elastic real area of contact versus dimensionless separation for contact with an elastic layer of t = 1 and 3μm in a range of the ratio El/E2

Fig. 8

Elastic contact load versus dimensionless separation for contact with an elastic layer of t=1 and 3μm in a range of the ratio El/E2

Fig. 9

Friction coefficient versus ratio of layer thickness to half contact width

Fig. 10

Friction coefficient and dimensionless contact area versus Θ  =  η0u/(p¯Ra) for various El/E2

Fig. 11

Friction coefficient versus speed u for various values of ψ

Fig. 12

Proportion of plastic asperities versus h/σ for various values of ψ

Fig. 13

Friction coefficient versus speed u for various values of η0

Fig. 14

Friction coefficient versus speed u for (a) experiments, (b) experiments (dots) against simulations (lines: α = 10 × 10−9 Pa−1 for Sim_Test 1, α = 4.3 × 10−9 Pa−1 for Sim_Test 2, and α = 3.0 × 10−9 Pa−1 for Sim_Test 3) at η0 = 6.625 mPa⋅s, and (c) simulations of A/An at η0 = 6.625 mPa⋅s

Fig. 15

Friction coefficient versus speed u for different test profiles

Fig. 16

(a) Friction coefficient versus speed u for different Ra: experiments (dots) against simulations (lines) and (b) hydrodynamic factor λ1 versus asperity factor λ2 for different Ra

Fig. 17

Contact with boundary-lubricated layered elastic solids

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